1. Field of the Invention
This invention relates to the field of foldable or elastomeric implantable intraocular lenses composed primarily of transparent silicone, and more particularly, pertains to a method for incorporating ultraviolet screening agents into such lenses.
2. Description of the Prior Art
Poly (methyl methacrylate), such as Perspex CQ from Imperial Chemical Industries (ICI), has been used for a number of years for intraocular lenses. Early implantable intraocular lenses were designed to replace cataract-damaged natural lenses, and were generally made of poly (methyl methacrylate). Ultraviolet screening agents were later added to the polymeric lens formulations because of concern over possible ultraviolet radiation damage to the retina.
Typical compounds used to impart ultraviolet protection to poly (methyl methacrylate) fall primarily into one of two classes, hydroxybenzophenones and their derivatives, or hydroxybenzotriazoles and their derivatives. These materials are available commercially under a number of different trade names and absorb different wavelengths of ultraviolet radiation depending on their substitute groups. Most of these compounds are effective in blocking transmission of 90% of the ultraviolet radiation below a wavelength of 400 nm when used at concentrations below 1% by weight in a plastic sheet 1 mm thick.
Transparent silicone lenses were developed later because the lenses allowed for insertion through a smaller incision by folding the lens.
Many of the same ultraviolet screening agents may be mixed into the silicone rubber compounds before they are fabricated into lenses and cured. However, in the interests of increased production volume, the silicone lenses are formed in mold cavities with the application of heat. These conditions produce an undesirable yellow color when ultraviolet screening agents are present. The screening agents also retard or inhibit the cure of some types of silicone resulting in undesirable weak and soft lenses. Additionally, when silicone lenses are treated to prevent fogging when immersed in water, some of the screening agents may be lost by leaching from the silicone matrix.
The shortcomings are minimized in the lenses of the present invention which are made by using diffusion to incorporate the ultraviolet screening agent into the lens after it has been shaped and cured.